Early formulations of the contraceptive pill may have been associated with an increased risk of breast cancer for some women, scientists have found.

However, they stress that the effect was only found among women at high risk of developing the disease.

They also stress that they have no compelling evidence to suggest that modern forms of the pill carry a similar risk.

The scientists, from the Mayo Clinic in the US, found that the women most at risk were those with a strong family history of breast cancer who had taken oral contraceptives prior to 1975.

What our data suggests is that it is early formulations that pose a risk, not later ones

Dr Thomas Sellers, Mayo Clinic

Lead research Dr Thomas Sellers said: "What our data suggests is that it is early formulations with high doses of oestrogen and progestins that pose a risk, not later ones.

"More importantly, these results don't apply for women at average risk for breast cancer, who should not interpret the study as reason to change their contraceptive practices."

The study, to be published in the Journal of the American Medical Association, found that the risk of breast cancer is 3.3 times greater for breast cancer patients' sisters and daughters who had ever used oral contraceptives compared to those with similar risks who had never used oral contraceptives.

This did not apply to nieces, granddaughters or women who married into the family, who only had a 1.2-fold greater risk of breast cancer with pill usage.

In families in which five or more blood relatives had been diagnosed with breast or ovarian cancer, the risk was even greater.

In those families, sisters and daughters of the breast cancer patients were 11.4 times more likely to develop breast cancer if they had ever taken oral contraceptives.

High doses

The elevated risk for first-degree relatives (sisters, daughters) of breast cancer patients was particularly evident for women who had used oral contraceptives introduced prior to 1975, when the formulations were more likely to contain higher doses of oestrogen and progestins.

Women who are currently taking oral contraceptive should not be alarmed and should not alter their contraception

Delyth Morgan, Breakthrough Breast Cancer

The study could not make statistically significant conclusions about sisters and daughters of breast cancer patients who had used more recent formulations of oral contraceptives containing lower doses of oestrogen and progestins, due to the small number of women in the study who had taken them.

This is the first multigenerational study of the association between oral contraceptive use and the development of breast cancer in women with a family history of the disease.

It included 426 families of women diagnosed with breast cancer between 1944 and 1952 at the Tumor Clinic of the University of Minnesota Hospital.

Participants included 394 sisters and daughters of the breast cancer patients, 3,002 granddaughters and nieces, and 2,754 women who married into the families.

Similar findings

Professor Valerie Beral, of the Imperial Cancer Research Fund's Oxford Unit, carried out a study of the effect of the pill on 50,000 women in 1996.

She found that taking the pill increased the risk of developing breast cancer among women at all levels of risk.

For instance, it was responsible for an extra 1.5 cases of breast cancer per 10,000 women who took the pill for five years during their early twenties.

However, she said: "The risk was small, and not sufficient to really make people change their behaviour."

Delyth Morgan, chief executive of the charity Breakthrough Breast Cancer, said: "Women who are currently taking oral contraceptive should not be alarmed and should not alter their contraception."